Batteries such as lead-acid batteries have been used for many diverse applications. For example, lead-acid batteries have been used as a starting, lighting and ignition power source for vehicles (SLI), as a power source for starting, lighting and other auxiliary power requirements in marine applications, and as a motive power source for use in golf carts and other electric vehicles. In addition, lead-acid batteries have been employed in a variety of stand-by power applications to provide a power source when the main power source becomes inoperable, as by, for example, interruption of electricity. Other representative applications for lead-acid batteries include uniform power distribution and power damping applications.
While the extent of discharge and the particular cycling requirements of a lead-acid battery for a specific application vary widely, one criterion remains constant. Specifically, it is important to ensure that proper charging of such batteries is carried out. Undercharging lead-acid batteries can result in less than optimum output and service life. For example, undercharging can result in perhaps permanent sulfation of part of the active materials, as well as stratification of the electrolyte and uneven use of the active materials.
On the other hand, undue overcharging of lead-acid batteries likewise can result in permanent damage to the batteries and can present potential safety hazards caused by, for example, dissociating the water in the electrolyte of the battery to gas. Further, overcharging lead-acid batteries can accelerate positive grid corrosion and even lead to bulging and/or buckling of the battery plates. Among other undesirable aspects of undue overcharging are an increase in the specific gravity of the electrolyte, possible oxidation of the separators and the undue heat generated that can accelerate various problems.
The time and manner in which lead-acid batteries are charged is also important. For example, many applications require charging within a relatively short period of time. In such circumstance, it is important to optimize the current or voltage used while, at the same time, avoiding the use of currents higher than the battery can accept for charging conversion.
U.S. Pat. No. 5,583,416 and U.S. Pat. No. 5,656,920, which are assigned to the same assignee as the present application and hereby incorporated by reference in their entirety, each disclose inventive methods and apparatus for charging batteries which avoid undercharging, overcharging and their associated adverse effects. For example, U.S. Pat. No. 5,583,416 discloses a method and apparatus for charging batteries which periodically applies voltage steps to the battery being charged to monitor the charging acceptance of the battery. At the beginning of the charging process, an initial target voltage is applied to the battery. The monitoring process includes increasing the applied voltage in two predetermined steps. The corresponding charging currents are measured at the initial applied voltage and at the two voltage steps. Based on these measurements, the current differentials (i.e., the current change for each voltage step range) are determined. The current differentials are compared to determine whether the increased voltage or the decreased voltage results in a more optimal charge acceptance, as indicated by a lower current differential. The target charging voltage is then adjusted in the direction of the smaller current differential, which indicates a more optimal charge acceptance. This monitoring process is repeated throughout the charging process to continuously adjust the charging voltage to approach a more optimized charge acceptance level. Alternatively, the charging current is controlled and stepped and the corresponding voltages are measured to determine the voltage differentials.
U.S. Pat. No. 5,656,920 discloses a charging method and apparatus which periodically applies voltage sweeps to the battery being charged to determine the optimal charging acceptance of that battery. Specifically, the charging voltage is "swept" across a range of values and the resulting current changes are measured. The range of the sweep is preferably from about the open-circuit voltage of the battery to just above the voltage region that provides the desired optimal charging performance. The current-voltage curve developed by this sweep is then analyzed to determine the charging voltage that corresponds to the optimal charging performance. The analysis of the current-voltage sweep curve is carried out, for example, by comparing the slope values at different points on the sweep curve to determine minimum or specific values. The voltage sweeps are performed periodically and the charging voltage is adjusted accordingly to provide optimized charging performance.
The methods disclosed in these two related U.S. applications provide satisfactory charging performances. However, these methods have their own system requirements. The step-charging method, for instance, requires a charging system which is capable of relatively precise voltage control so that the current differentials can be accurately determined. The voltage-sweep method also requires precise voltage control and is best suited for applications wherein the electrical system powered by the battery is relatively insensitive to the relatively large voltage changes caused by the voltage sweeps.